As a recordable optical recording medium corresponding to the specifications of a compact disk (hereinafter simply referred to as a “CD”), a CD-R (CD-Recordable) has been widely spread. The storage capacity of the CD-R is about 680 MB. However, with a drastic increase of the information volume, demands for higher density and larger storage capacity in an information recording medium have been increased.
As a means for achieving higher density of a recording medium, it is considered to decrease a beam spot by using a shorter-wavelength laser in writing and reading and increasing a numerical aperture (N.A.) of an object lens. As the short-wavelength laser for use in an optical disk system, red laser beams of 500 to 700 nm, further 630 to 690 nm, more specifically, 680, 670, 660, 650, and 635 nm have been put to practical use. Thus, by virtue of techniques for reducing the wavelength of a semiconductor laser, increasing the numerical aperture of an object lens, and compressing data, an optical recording medium capable of recording a motion picture and large volumetric information has been successfully produced. Examples of optical recording media proposed up to the present include a magneto-optical recording medium, phase change recording medium, chalcogen oxide-based optical recording medium, and organic dye-based optical recording medium. Of them, the organic dye-based optical recording medium is considered preferential in view of low cost and easy processing. In consideration of these circumstances, the one developed as an optical recording medium capable of recording and regenerating a motion picture with the same quality level as that of TV and with a higher density than that of CD and as a recordable optical recording medium capable of regenerating by a commercially available DVD video player or a DVD-ROM player that have been widely used and capable of recording by a red semiconductor laser having an oscillation wavelength of 630 to 690 nm is a recordable digital versatile disc (hereinafter, simply referred to as a “DVD-R”). DVD-R is a write-once optical recording medium having a 3.9 or 4.7 GB storage capacity. In particular, only recently, a DVD-R medium having a single-side capacity of 4.7 GB has been brought into a market. Such a DVD-R medium also employs a stacked structure, which is formed of a recording layer containing a cyanine-based dye, azo-based dye, or the like, and a reflecting layer, is characterized by a disk structure formed by adhering two substrates of 0.6 mm thick. Regarding such an optical disk having satisfactory recording characteristics suitable for this capacity, development of a medium applicable to high-speed recording has been aggressively made at present.
Furthermore, it has been estimated that a further higher density recording will be desired in future and that the amount of data stored in a single disk will reach up to 15 to 30 GB. To attain such a recording density, a further shorter-wavelength laser will be inevitably used. Accordingly, as a recording dye suitable for an on-coming organic dye-based optical recording medium, it is desired to develop a dye having good recording characteristics within the wavelength range of 300 to 500 nm.
In the meantime, regarding a medium attaining high-density recording more excellent than DVD-R using an organic dye in a recording layer, Japanese Patent Laid-Open No. 10-302310 discloses that a recording density having a storage capacity of 8 GB or more is attained by using a laser having an oscillation wavelength of 680 nm or less. This publication suggests that a large storage capacity of 8 GB or more should be attained by converging laser light of 680 nm or less through an object lens having a high numerical aperture of 0.7 or more via a light transmission layer of 10 to 177 μm thick.
On the other hand, only in recent years, as a blue laser having an oscillation wavelength of 390 to 430 nm, a laser of 410 nm using a GaN-based material and an SHG laser of 425 nm wavelength, which is obtained by combining a semiconductor laser and an optical wave guide element, have been developed (for example, January 26 Issue of Nikkei Electronics, No. 708, p. 117, 1998). Thus, development of blue semiconductor laser applicable dyes suitably used in such a laser is now proceeding.
Furthermore, Nichia Corporation started supply of a GaN-based semiconductor laser emitting bluish-violet light having an oscillation wavelength of 400 to 410 nm from the beginning of 1999. Since then, studies on a medium having a high-density storage capacity of 15 GB or more per side (hereinafter referred to as an “HD-DVD-R medium”) and capable of recording a motion picture having the same image-quality level as that of a high definition television (HDTV) for about 2 hours have been started. Such an HD-DVD-R medium is capable of recording motion pictures for about 6 hours with the same image-quality level as those currently on air and therefore attracted attention also as a new recording medium in place of a home VTR. Already now, technical outline of a proposal for a medium using an inorganic recording film of a phase change system has been introduced in September 6 Issue of Nikkei Electronics, No. 751, p. 117 (1999).
The organic dye compounds for use in recording by a blue laser of 400 to 500 nm currently proposed include cyanine-based dye compounds described in Japanese Patent Laid-Open Nos. 4-74690, 6-40161, 2001-232945, 2001-246851, 2001-260536, and 2001-301333; porphyrin-based dye compounds described in Japanese Patent Laid-Open Nos. 7-304256, 7-304257, 8-127174, 11-334207, 2001-39032, 2001-80217, 2001-84594, 2001-138633, 2001-138634, 2001-143317, 2001-180117, 2001-181524, and 2001-287462; polyene-based dye compounds described in Japanese Patent Laid-Open Nos. 4-78576 and 4-89279; azo-based dye compounds described in Japanese Patent Laid-Open Nos. 11-334204,11-334205 and 2001-271001; dicyanovinylphenyl dye compounds described in Japanese Patent Laid-Open No. 11-304206; coumarin compounds described in Japanese Patent Laid-Open Nos. 2000-43423 and 2001-96918; pyrimidine compounds described in Japanese Patent Laid-Open No. 2000-163799; naphthalocyanine compounds described in Japanese Patent Laid-Open No. 2000-228028; five-membered hetero ring compounds described in Japanese Patent Laid-Open No. 2000-335110; bis-azole compounds described in Japanese Patent Laid-Open No. 2000-343824; amino pyridine compounds described in Japanese Patent Laid-Open No. 2000-343825; bis-pyridinium compounds described in Japanese Patent Laid-Open No. 2001-63211; oxonol compounds described in Japanese Patent Laid-Open Nos. 2001-71638 and 2001-328351; styryl compounds described in Japanese Patent Laid-Open Nos. 2001-71639 and 2002-2110; amino butadiene compounds described in Japanese Patent Laid-Open No. 2001-146074; metal chelate compounds described in Japanese Patent Laid-Open Nos. 2001-158862, 2001-214084, and 2002-36727; quinone or quinodimethane compounds described in Japanese Patent Laid-Open No. 2001-232944; hydrazone compounds described in Japanese Patent Laid-Open No. 2001-234154; triazine compounds described in Japanese Patent Laid-Open No. 2001-277720; carbostyryl compounds or naphthylidine compounds described in Japanese Patent Laid-Open No. 2001-287466; condensed heterocyclic compounds described in Japanese Patent Laid-Open No. 2001-301329; and stilbene compounds described in Japanese Patent Laid-Open No. 2002-2117.
Further, the optical recording media are proposed including an optical recording medium described in Japanese Patent Laid-Open No. 11-53758, which is formed of two layers: one is a recording layer primarily containing a porphyrin-based dye or cyanine-based dye as an organic dye for forming the recording layer and the other is a metal reflecting layer primarily containing silver; an optical recording medium described in Japanese Patent Laid-Open No. 11-203729 which attains writing in 2 wavelength regions by devising the constitution of a medium, that is, using a medium having a blue laser responsive dye layer containing a cyanine-based dye responsible to a blue laser and a red laser responsive dye layer; an optical recording medium using an indigoid-based dye compound described in Japanese Patent Laid-Open No. 11-78239 which attaining writing in two wavelength regions by mixing two types of dyes, that is, a dye for a blue laser and a dye for a red laser; an optical recording medium using a cyanoethene-based dye described in Japanese Patent Laid-Open No. 11-105423; and an optical recording medium using a squarylium-based dye compound described in Japanese Patent Laid-Open No. 11-110815.
On the other hand, as examples for using an organic dye film for writing within the blue region of 400 to 500 nm, Japanese Patent Laid-Open Nos. 7-304256 and 7-304257 have suggested to mix a molecular compound coordinated to the central metal of a porphyrin-based compound and a polymer or a polymer having a molecular structure having a central metal coordinated as a side chain, thereby shifting the Soret zone of the porphyrin-based compound toward a long wavelength range so as to correspond to an Ar laser of 488 nm, and simultaneously to reduce manufacturing cost by achieving film formation by spin coating. Furthermore, Japanese Patent Laid-Open Nos. 4-78576 and 4-89279 disclose polyene-based color compounds, which are, however, poor in light stability according to the studies by the present inventors, and required some modifications, for example, blending of a quencher or the like, to put them to practical use.
As the recent circumstances, since the prospect of putting a bluish-violet semiconductor laser of 400 to 410 nm wavelength to practical use is given, development of a high-capacity recordable optical recording medium using the laser has been aggressively made and particularly development of dyes having excellent light stability and high-speed recording characteristics has been desired.
However, the optical recording media mentioned above are actually insufficient to be subjected to the laser light having a wavelength of 400 to 410 nm. More specifically, the media using the organic dyes mentioned above have a problem in that when recorded signal data is read out, the read-out of a signal is not satisfactory performed, since the ratio of carrier to noise (C/N) is not always proper. Therefore, development of an optical recording medium capable of overcoming this problem and writing and reading high-density data by a laser beam of 400 to 410 nm wavelength has been urgent need.
As a result of studies conducted by the present inventors on recording materials suitable for an optical recording medium, the following two findings were obtained.
(1) Since a large capacity optical recording medium uses a laser beam of 300 to 500 nm in writing and reading data, it is important to control the absorption coefficient, refractive index, and reflectivity of the recording medium in the vicinity of the laser wavelength.
(2) As mentioned above, although large-capacity optical recording media using the laser have been aggressively developed and particularly the development of a dye having excellent light stability and good high-speed recording characteristics has been desired, the dye compounds mentioned above have not yet satisfied recording characteristics as a recording material capable of reading and writing data with a laser beam of the wavelength range and thus still need to be improved at present. Furthermore, as a favorable characteristic of a dye for use in a medium manufactured by a coating method, such as spin coating, which is a simple method for forming a recording film, the high solubility of the dye to a coating solvent must be considered.
Also, in general, to increase the storage capacity, higher-density recording must be performed. To attain this, it is necessary to increase the numerical aperture of an object lens in order to converge an optical beam for use in recording and generate a shorter-wavelength laser by an optical system. However, when an optical beam is converged, the minimum beam diameter is determined by its diffraction-limit.
In the meantime, since writing is made when the intensity of beam exceeds a certain threshold, the record pit obtained is smaller than the beam spot converged, as shown in FIG. 1(a). The periphery of the record pit corresponds to the skirts of the intensity peak. Under the present circumstances where a further shorter wavelength laser is developed, a photochemical reaction in the recording layer is facilitated even in the periphery of the record pit. In particular, in the wavelength range of the aforementioned bluish-violet laser, a photochemical reaction of an organic compound is likely to occur, producing a problem: a pit edge is degraded during writing time, decreasing signal characteristics. To explain more specifically, recoding data which must be essentially written in response to a rectangular wave [indicated by a solid line in FIG. 1(b)], as shown in FIG. 1(b), results in a broader tailed wave form (indicated by a broken line in FIG. 1(b) due to the deterioration of the pit edge. Furthermore, when regeneration is performed by the same bluish-violet laser wavelength as in recording, a photo reaction is facilitated even by weak light irradiation such as regeneration light. As a result, deterioration proceeds every time regeneration is performed. Against such a problem, Japanese Patent Laid-Open Nos. 7-304256 and 7-304257 take measures by varying the wavelength of recording light from that of regeneration light, virtually, using a longer wavelength light as regeneration light than recording light. As a result, the requirement for high-density recording has not yet sufficiently satisfied up to present. Furthermore, using recording light and regeneration light different in wavelength means that a recording device and a regeneration device must be separately prepared or means that two optical systems and control systems must be provided in a single apparatus, with the result that such an optical recording medium is limited in use and entails enlargement of the apparatus, increase of cost, and losing general versatility. Furthermore, in a conventional optical recording medium such as CD-R, on/off of writing can be determined based on whether or not reaching thermal threshold on physical characteristics such as melting point, sublimation point, phase transition point or thermo-decomposition point of an organic dye film. However, such a contrast is made indistinctive by optical deterioration due to excitation of a bluish-violet laser. In particular, in a high-density recording system in which recording pits must be formed smaller than that of an optical beam, there is a possibility that the quality of a recording signal may significantly decrease.
Now, as a case where an organic dye film is used in recording in the visible light range of 400 to 700 nm, Published Japanese translations of PCT international publication No. 2002-501497 describes a writable and erasable optical recording medium using a perylene-based compound. Further, Japanese Patent Laid-Open No. 10-6645 describes a laser beam of 620 to 690 nm wavelength used on a medium using a perylene compound analogue in the recording layer. However, it was found that these compounds are still insufficient with respect to light stability and signal characteristics when a bluish-violet laser of 400 to 410 nm wavelength is used for writing.
Also, as an optical recording medium using an organic dye, on which recording can be made by a laser of 400 to 500 nm wavelength, Japanese Patent Laid-Open No. 2000-113504 describes an optical recording medium using a naphthalene imide compound. However, it was found that the compound was still unsatisfactory in light stability and the recording medium thus required further improvement.